Reference is made to FIG. 1 showing a cross-section of a conventional backside illuminated photosensor element 10. The illustration in FIG. 1 is of just a single element (also referred to as a pixel), it being understood that an image sensor is formed of a plurality of such photosensor elements typically arranged in an array. The element 10 comprises a semiconductor substrate 12 that is doped with a first conductivity type dopant (for example, a p-type dopant) with a dopant concentration of 1×1015 to 5×1016 at/cm3. The semiconductor substrate 12 may, for example, be made of silicon and have a thickness of 3-10 μm. In an embodiment, the semiconductor substrate 12 may comprise the upper silicon layer of a silicon-on-insulator (SOI) substrate that is produced in a process where a handle is attached to the front of the SOI substrate and the back of the SOI substrate is thinned to remove the carrier substrate and buried oxide layers.
At the front surface 14 of the semiconductor substrate 12, a region 16 is provided for trapping photogenerated carriers and charge transfer components. The region 16 is doped with a second conductivity type dopant (for example, an n-type dopant) with a dopant concentration of 1×1017 to 1×1019 at/cm3. A circuit 18 for transferring photogenerated charge is also provided at the front surface 14. The circuit 18 comprises a metal oxide semiconductor (MOS) transistor including an insulated gate 20 and a read region 22. The read region 22 is doped with the second conductivity type dopant with a dopant concentration of 1×1019 to 1×1021 at/cm3. The insulated gate 20 extends over a channel portion of the substrate 12 between the regions 16 and 22 which form source-drain regions of the MOS transistor. An insulating region 30 including interconnection circuits formed of conductive tracks 32 and vias 34, commonly referred to in the art as metallization levels, is further provided on top of the front surface 14. The conductive tracks 32 of the interconnection circuits may be electrically coupled to the insulated gate 20, read region 22 and substrate 12 body using contact structures (not shown) such as, for example, tungsten plugs.
Each photosensor element includes a pixel substrate (active) region 40 formed by a portion of the substrate 12 that includes the regions 16 and 22. The pixel substrate region 40 is delimited by a surrounding isolation structure 42 that extends from the front surface 14 to the back surface 44 of the semiconductor substrate 12. The isolation structure 42 includes a core region 46 formed of a reflective metal material such as, for example, aluminum, titanium, titanium nitride, titanium alloy, tungsten, chromium, copper, and the like. The isolation structure 42 further includes a peripheral region 48, lining the core region 46 and further a dielectric peripheral region 49 lining the region 48 and positioned between the core region 46 and the pixel substrate region 40. The peripheral region 48 may, for example, be doped with the first conductivity type dopant with a dopant concentration of 1×1018 to 1×1020 at/cm3. The peripheral region 49 may comprise, for example, silicon oxide or silicon nitride material.
A layer 50 made of an antireflection dielectric coating (ARC) material is mounted to the back surface 44 of the semiconductor substrate 12. The ARC material may, for example, comprise a resin, an organic polymer, a material including silicon and nitrogen, a material including silicon and oxygen, or a material including silicon, oxygen and nitrogen. A color filter layer 52 is mounted to the antireflection layer 50. The color filter layer 52 may be configured to filter a particular range of electromagnetic wavelengths in the visible and/or infrared range as desired for the particular application of the element 10 and its associated imaging array. A microlens 54 is mounted to the color filter layer 52. The optical axis 56 of the microlens 54 may be aligned with a center 58 of the pixel substrate region 40 as delimited by the surrounding isolation structure 42.
In operation, a ray or photon 59 is collected by the microlens 54 and passes through the color filter layer 52 and antireflection layer 50 to enter the pixel substrate region 40. The ray or photon 59 may generate an electron/hole pair in the semiconductor substrate 12. The electron originating from the received ray or photon 59 is then collected in the region 16. With the application of an appropriate voltage to the insulated gate 20, the MOS transistor of circuit 18 is turned on and the charge collected in region 16 is transferred to read region 22.
Depending on the angle of incidence of the collected ray or photon 59, the ray or photon 59 may pass through the pixel substrate region 40 and be reflected by the core region 46 of the isolation structure 42 and directed towards the front surface 14 of the semiconductor substrate 12 as generally shown at reference 60. This reflected ray or photon 60 may generate an electron/hole pair in the pixel substrate region 40. The electron originating from the reflected ray or photon 60 is then collected in the region 16. With the application of an appropriate voltage to the insulated gate 20, the MOS transistor of circuit 18 is turned on and the charge collected in region 16 is transferred to read region 22. This accordingly produces an increase in sensitivity of the element 10.
It is further known that an electric field produced by the isolation structure 42 functions to force free electrons generated from the ray or photon received in the pixel substrate region 40 towards the center 58 of pixel substrate region 40 where they are more readily captured in the region 16. This further improves the sensitivity of the element 10.
The element 10 further comprises a reflecting metal layer 66 in the insulating region 30. This reflecting metal layer 66 may, for example, be formed by same material as the conductive tracks 32 of the interconnection circuits at a relatively lower one of the plural metallization levels. In an embodiment, reflecting metal layer 66 may be one of the conductive tracks 32 (i.e., it is not a floating conductive element). The reflected ray or photon 60 may pass completely through the semiconductor substrate 12 and insulating region 30 to be further reflected back into the pixel substrate region 40 of the semiconductor substrate 12 by the reflecting metal layer 66 as generally shown at reference 68. The further reflected ray or photon 68 may generate an electron/hole pair in the pixel substrate region 40. The electron originating from the further reflected ray or photon 68 is then collected in the region 16. With the application of an appropriate voltage to the insulated gate 20, the MOS transistor of circuit 18 is turned on and the charge collected in region 16 is transferred to read region 22. This accordingly produces an increase in sensitivity of the element 10.
Notwithstanding the improvements in sensitivity produced by presence of the isolation structure 42 and reflecting metal layer 66, there remains a need for further sensitivity improvement accomplished by increasing opportunities for rays or photons received by the photosensor element to produce electron/hole pairs.